In order to study the effect of different silicon content on the structure and properties of ductile iron and the mechanism of silicon on high silicon ductile iron, one group of QT450-10 ductile iron samples was designed as the control group, and three groups of high silicon ductile iron samples with silicon content in the range of 3.5%~4.5% were designed as the test group. According to the calculation formula of carbon equivalent:
ω (CE)%= ω (C)%+0.3[ ω (Si)%+ ω (P)%]
While keeping the carbon equivalent basically unchanged, the silicon content increases and the carbon content decreases relatively. 1 t medium frequency induction furnace is used for smelting, and EPC process is used for casting. EPS material is used for foam white mold, the main component of coating is bauxite, the molding sand is dry silica sand, the pouring temperature is 1496 ℃, the negative pressure is 0.05 MPa, and the pouring time is 30 s. Wire feeding spheroidizing method is used for spheroidizing, the addition amount of spheroidizing agent is 1.3%, and the addition amount of inoculant is 75 ferrosilicon, 1.5%~2.5%. The wall thickness of the foam white mold Y-shaped sample is 24 mm, and the size is determined according to the national standard GB/T 1348-2019 Nodular Iron Castings. The chemical composition of the final four groups of samples is shown in the table.
| Sample No | C | Si | Mn | P | S | Mg(residue) | RE | CE |
| A | 3.52 | 2.92 | 0.183 | 0.029 | 0.0058 | 0.064 | 0.0160 | 4.4 |
| B | 3.09 | 3.68 | 0.273 | 0.027 | 0.0057 | 0.068 | 0.0155 | 4.2 |
| C | 2.88 | 4.22 | 0.311 | 0.026 | 0.0037 | 0.059 | 0.0155 | 4.15 |
| D | 2.60 | 4.59 | 0.265 | 0.024 | 0.0046 | 0.057 | 0.0145 | 3.98 |
During production, the silicon content of as cast QT450-10 ductile iron should be controlled at 2.5%~3.0%, and the silicon content of group A sample should be 2.92%, meeting the production requirements; According to the Fe-C phase diagram, under equilibrium conditions, when the carbon equivalent is 4.34%, it is eutectic composition, but the change of silicon content will shift the eutectic point and change the eutectic composition. Therefore, the carbon equivalent of the ductile iron sample is controlled within the range of 4.0%~4.4%; The contents of manganese, phosphorus, sulfur, magnesium and other elements in the four groups of samples are roughly at the same level. To sum up, the chemical composition of QT450-10 and high silicon ductile iron samples obtained by pouring meet the requirements of experimental design.
Machining the as cast sample to 10 mm × 10 mm × After grinding and polishing, the 10 mm metallographic sample was corroded with 4% nitric acid alcohol solution for 10~15 seconds. The microstructure was observed with Olympus DSX500 optical microscope (OM) and ZEISS Ultra Plus field emission scanning electron microscope (SEM), and ipp-6.0 and OLYCIA The m3 image analysis software statistically measures the relative content and size of ferrite and graphite, and grades the graphite morphology and size according to GB/T 9441-2021 Metallographic Inspection of Ductile Iron. After metallographic observation, use a digital Brinell hardness tester and Vickers hardness tester to test the macro hardness of the sample. The Brinell hardness test load is 750 N, and the Vickers hardness test load is 300 N. For mechanical properties, AG XPLUS universal testing machine is used for tensile test of the sample. The diameter of the parallel section of the sample is 14 mm, the gauge length is 70 mm, and the length is 180 mm.